Fix a race condition that could lead to callbacks for newly added notifiers not being called
Fix a race condition that could lead to callbacks for newly added notifiers not being called

There was a window where notifers were in neither m_notifiers nor
m_new_notifiers and the mutex was unlocked, which could let the main thread
incorrectly think there were no notifiers that it needed to wait for or call.

realm/realm-object-store,realm/realm-core,realm/realm-object-store,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-object-store